Area Light vs. Emissive Polygon
Matt's answer covers the theory of it, but to provide a visual comparison:


Area lamp, 100 samples, 11.08 seconds. MIS (Multiple Importance Sample) on


Plane, 100 samples, 12.10 seconds. MIS on

So the plane is slightly slower, but it has slightly less noise as well.

But now lets try with three lamps instead of only one:




Area lamps, 100 samples, 11.31 seconds, MIS off for all lamps (turning MIS on gave the same result with an extra 2 seconds on the render time)


Planes, 100 samples, 12.83 seconds, MIS on for all planes


Planes, 150 samples, 11.19 seconds, MIS off for all planes


Planes, 100 samples, 12.46 seconds, MIS on for two smaller planes, off for large one

So it appears that when you have multiple lights and manage which emission planes have MIS turned on, the result can me similar to area lamps, although still a bit slower and slightly more noisy. It's the other way around when you have only one emission plane, but the difference is subtle. MIS for area lamps doesn't seem to make any difference to the diffuse, except increase the render time a bit.

This only compares noise level of shadows and diffuse shading, for Glossy shaders you usually want to keep MIS on for the lamps you wish to create specular highlights. (See page 5 of this paper):



(From the above mentioned paper)

Now for arguement's sake, lets take a look at a point lamp vs a spherical mesh:


Point Lamp, 100 samples, 12.30 seconds, MIS on


Sphere, 100 samples, 13.86 seconds, MIS on

The sphere is clearly the loser here, very noisy and slightly slower. Turning MIS off makes it much worse.

To conclude - it all depends on your situation. Mesh lights are more flexible and probably more accurate, but require some understanding of importance sampling and playing around.

All these tests were done with the Path Tracing integrator. Using the Branched Path Tracing integrator, Area and Point lamp renders were much cleaner, and mesh lamp renders were much noisier (for various samples but the same render time).

Download this .blend file if you'd like to play with it.